Conversion of non-petroleum feedstocks into liquid fuels, such as jet fuel, diesel fuel, or gasoline, has long been of interest due to the limited distribution of petroleum reserves and due to the possibility of producing liquid fuels from biomass and/or waste, which can reduce or eliminate lifecycle CO2 emissions for liquid fuels. Current technologies for converting carbonaceous material into liquid fuels tend to fall into two categories: 1) pyrolysis followed by hydrotreating; and 2) gasification followed by Fischer-Tropsch synthesis.
In the first category are processes in which the carbonaceous material is pyrolyzed by heating the feed to temperatures of 500-600° C. in the absence of molecular oxygen. This results in the generation of solid and gaseous products. The solid product, called char and/or ash, is separated from the gaseous products. The gaseous products are cooled to room temperature to condense a portion of the gaseous products into a liquid that superficially resembles crude oil called pyrolysis oil, or when the carbonaceous material is biomass, bio-oil. The pyrolysis-oil is then further processed to produce a liquid fuel.
However, when the carbonaceous material contains oxygen—as is the case for materials such as coal, biomass, municipal solid waste, etc.—a significant amount of oxygen is incorporated into the molecules of which the pyrolysis oil is comprised. The oxygen in the pyrolysis oil is often manifested in functional groups such as hydroxyl and carboxylic-acid groups. The presence of these compounds in the pyrolysis oil results in a pyrolysis oil that has a low heating value, a very high acidity, and lacks stability. The low heating value reduces the value of the pyrolysis oil as a fuel or feedstock for producing fuel, the high acidity makes it incompatible with the existing petroleum infrastructure, and the instability results in excessive gum formation and increases in viscosity when stored at or above room temperature.
In order to address these issues with pyrolysis oil, it is often hydrotreated with hydrogen to remove the oxygen. This results in a pyrolysis oil that has acceptable properties at mass yields of up to 30% depending upon the feedstock and if the source of hydrogen is natural gas. However, the production of hydrogen requires additional equipment operating at high temperatures and pressures, and, in some cases, a source of water. These requirements increase the cost of producing a marketable product. Also, depending upon the source of the fuel used to produce the hydrogen, there may be an increase in fossil-fuel-derived CO2 emissions or a reduction in yield.
In the second category are processes where the carbonaceous material is gasified at temperatures of 800-1000° C. to produce synthesis gas (mainly CO and H2). Because gasification yields a composition close to equilibrium, the specific chemical makeup of the feed only affects the H:C and C:O ratios in the synthesis gas, thus making control of the composition of the final product straightforward. After removing contaminants from the hot gas, a Fischer-Tropsch (F-T) process is employed to link C1 compounds (e.g., CO, CH4, etc.) to produce a targeted hydrocarbon product such as synthetic JP-8 jet fuel. This makes a synthetic fuel that is very close to the desired product. However, this approach is very expensive due, in part, to both the catalytic F-T process, but also due to the complexities of hot-gas clean-up.
There is a continuing need for technologies that convert carbonaceous material to a synthetic fuel at a cost competitive with traditional petroleum-derived fuels.